1. Field of the Invention
The present invention is directed to a directly cooled magnetic coil, particularly a gradient coil for a magnetic resonance apparatus. The invention is also directed to a method for manufacturing conductors for a directly cooled magnetic coil.
2. Description of the Prior Art
In order to assure operation below a maximally allowed temperature of the gradient coil, it is necessary to designationally and efficiently eliminate the electrical dissipated power that arises in the form of heat. Since a dissipated power on the order of magnitude of more than 20 kW can be involved, considerable demands are made on the cooling system employed for this purpose.
German OS 34 45 448 discloses a gradient coil that is composed of two individual coils electrically connected in series. The individual coils are arranged in housing walls of a coil housing that lie opposite one another such that a free space remains for use as a cooling channel between the individual coils.
European Application 0 896 228 discloses an actively shielded gradient coil system with primary and shielding coils. Cooling tubes are located between these coils.
U.S. Pat. No. 5,068,491 discloses a rigid conductor for a power supply with coolant channels.
German OS 25 44 275 discloses a fluid-cooled induction coil. At least one further, inner waveguide is arranged within an outer waveguide. The interspace between the outer and inner waveguide has a fluid therethrough.
The provision of a cooling channel in the electrical conductor for the turns of the gradient coil requires a complete insulation of the coolant circulation path due to the high, and partially different voltages, in the region of the gradient coil and, given employment of water as the coolant, requires the use of highly distilled, non-conductive (i.e., non-ionized) water, which, of course, makes the operation extremely complicated and expensive. Given the high voltages and the direct contact between the water and the metallic conductors, an ionic contamination occurs after a relatively short time, and thus, the water becomes conductive, which absolutely must be avoided because of the risk of high-voltage arcing.
An object of the present invention is to provide a directly cooled magnetic coil of the type initially described wherein a simple operation is possible using non-processed water, as well as to provide a method for manufacturing conductors for such a directly cooled magnetic coil.
These objects are inventively achieved in a magnetic coil wherein the conductors are fashioned as profiled segment conductors whose individual profiled segments surround a cooling tube made of material that is electrically non-conductive, or only slightly conductive, particularly flexible plastic.
Due to the inventive structure of the coil, the cooling fluid is completely isolated from the inside wall of the electrical conductor to be cooled, so that if water is used as the coolant, it is not important whether the water has a more or less high electrical conductivity due to natural contaminants. A quasi-direct cooling of the conductors of the coil windings by the coolant takes place that is only slightly impeded by the small wall thickness of the electrically insulating cooling tubes, so that a very efficient cooling is established that opposes an excessive heating of the magnetic coil. Due to the small space requirement of an inventively constructed gradient coil compared to known versions with direct cooling and high, complicated outside insulation, or indirect cooling with cooling coils surrounding the gradient turns, the gradient coil windings can be positioned at more extreme radial distances, allowing an additional efficiency gain in the field generation. The elimination of water conditioning measures for lowering the specific electrical conductivity and for minimizing corrosion effects on the conductor material simplifies the operation of an inventively constructed, directly cooled magnetic coil and makes it less expensive.
The structure of the conductors for an inventive, directly cooled magnetic coil can include as the conductor segments, individual strands twisted with one another to form a stranded conductor that surrounds the cooling tube. The manufacture of such a conductor can preferably ensue by spinning the individual conductors around the cooling tube.
A design which has proven particularly advantageous in extensive tests on which the present invention is based has profiled segments in the form of rectangular (in cross-section) rods each having a channel with a semicircular cross-section, surrounding the cooling tube and positively locked thereto. Such a structure can be manufactured in an extremely simple way, since the rectangular conductors correspond to the profile shape that is standard for highly stressed gradient coils and can be easily laid via templets. Further individual conductors, i.e. solid conductors as well as stranded conductors, can be applied onto the outside surfaces of the profiled segments in order to enhance the effective conductor areas.
According to a further embodiment of the invention, at least some of the conductor segments applied at the cooling tube can be at in the form of retaining, embracing mounting webs which are mounted to, and at least partially surround, the cooling tube. For example, longitudinal webs which are offset relative to one another by 180xc2x0 can be applied to the cooling tube with a number of hooked cross-webs, spaced from each other, being attached to each longitudinal web. This embodiment yields a flexible plastic part, so that bending of the finished conductor is possible without problems. The actual conductor segments composed, for example, of copper can, of course, already be bent with good shape retention.
In a further embodiment of the invention, lateral tube profiles which respectively accept a profiled segment in a positively locked fashion are applied to the cooling tube, which in this embodiment preferably has a rectangular cross-section. These lateral tube profiles also preferably have a rectangular cross-section and form a smooth, rectangular outside contour. As warranted, parallel, spaced slots can thereby be provided in the walls of the lateral tube profiles that form the outside contour in order, similar to the exemplary embodiment described above having the spaced cross-webs, to enable a simple, shape-retaining bending of the finished conductors.
The manufacture of such coil conductor having profiled segments arranged in lateral tube profiles of the cooling tube can ensue very simply by coating or extrusion processes using thermoplastic plastics.